Photocatalysis is a natural electronic process brought about by the absorption of UV or visible radiation at the surface of a substance, referred to as photocatalyst. By using light energy, the photocatalysts bring about the formation of free radicals capable of decomposing, by oxidation/reduction, certain organic or inorganic substances present in the medium in which they are immersed. Thus, the main advantage of photocatalysis lies in the fact that the energy necessary for the oxidation/reduction reactions is supplied by direct absorption of light, rather than by thermal heating.
The photocatalysts used are semiconducting materials having a forbidden band (optical gap), typically of between 3 and 4 eV, corresponding to irradiation with light in the spectral region of the near UV. The absorption of photons with an energy greater than the optical gap results in the formation of electron-hole pairs within the semiconductor, it being possible for these charge carriers subsequently either to recombine according to various mechanisms or to diffuse to the surface of the semiconductor.
Thus, the photocatalytic reaction which takes place at the surface of a semiconducting material comprises several stages,                the adsorption of the reactants at the surface of the photocatalyst,        the formation of electron-hole pairs by absorption of photons resulting from irradiation with UV,        the separation of the electron-hole pairs and their migration to the surface of the photocatalyst,        oxidation and reduction reactions of the electrons and holes with other adsorbed entities, such as pollutants, pollen, bacteria or viruses, resulting in the decomposition of these entities, and        the desorption of the reaction products.        
The rate at which the photocatalysis reactions take place depends on the light intensity, on the amount of photocatalyst (number and lifetime of the charge carriers) and on the duration of the contact between the semiconductor and the materials present in the medium in which they are immersed.
The most commonly used photocatalysts are wide-gap semiconductors based on oxides or sulfur, such as TiO2, ZnO, CeO2, ZrO2, SnO2, CdS or ZnS, the most widely used photocatalyst being titanium dioxide (TiO2) due to its thermodynamic stability, its absence of toxicity and its low cost.
Thus, the active oxygen originating from the photocatalytic reaction is capable of decomposing and destroying:                volatile organic compounds (VOCs),        NOx gases escaping from vehicles and factories,        bacteria, viruses, microbes,        molds, algae, fungi,        allergens, such as pollen and acarids,        human, animal and chemical odors.        
Photocatalysis is thus used in the field of water treatment, air treatment and deodorizing but also as antibacterial agent. Photocatalysis may also find applications in the medical field for combating infected cells.
In industry, the principle of photocatalysis is also employed in the use of self-cleaning glass, this application being associated with a second property of the irradiated semiconductor: superhydrophilicity. Because of its special microscopic coating, a self-cleaning glass has the ability to decompose organic contaminants and thus to remain clean longer than an ordinary glass. The process for the manufacture of self-cleaning glasses comprises a stage of application, to its external face, of a special photocatalytic layer based on titanium dioxide (TiO2). The self-cleaning function of these glasses is based on the union of two properties of the microscopic layers deposited: photocatalysis and superhydrophilicity. This is because the hydrophilic properties of this glass mean that the water falling on the glass sheet washes the glass, instead of leaving it dirty, like an ordinary glass. Instead of falling as drops on the glass, the water gradually forms a film which, by gravity, ends up by sliding along the glass while washing it. Thus, self-cleaning glasses make possible a reduction in the cleaning costs but also in the environmental impacts as they require a reduced use of detergents.
Current photocatalytic materials are mainly manufactured according to expensive sol-gel processes requiring the use of precursors (Srivastava et al., International Journal of Hydrogen Energy, Vol. 25, pp. 495-503, 2000).
Photocatalysts of high porosity have been prepared from composite materials composed of titanium and aluminum, the composite material deposited being subjected to an electrochemical anodization, the aluminum oxide formed subsequently being removed by attacking with a solution of strong acid H3PO4 (5%) and CrO3 (2%) at a temperature of 80° C., the use of concentrated acid baths being very problematic to handle (Phys. Stat. Sol., No. 12, 3690-3693 (2008)). Furthermore, the deposition and thermal annealing temperatures employed in this process do not make it possible to achieve a satisfactory photocatalytic activity.
Another disadvantage of the known methods of the state of the art results from the low porosity and the low specific surface of the photocatalysts obtained, the latter in fact exhibiting an insufficient effectiveness, the photocatalytic efficiencies of these materials being between 0.5 and 3%.
Thus, the technical problem remaining to be solved, with respect to this state of the art, consists of the development of a photocatalyst which has an improved photocatalytic activity, which has excellent adhesion to the substrate and which can be applied as thick layers. This photocatalyst should also be able to be employed according to processes which are simple and economic and which are highly feasible industrially.